The truck stopped only when it smacked into at least five cars and a traffic-light post. An elderly man was killed last night and eight people were injured when a speeding truck ran a light and plowed into pedestrians in a crosswalk on one of the busiest corners of the Upper West Side, police said.

A police source told The Post the mayhem occurred about 9 p.m. after officers in a squad car saw the white pick-up driving erratically and followed it to Broadway and West 72nd Street.

The source said the officers were not “chasing” the car at a high rate of speed. It was not immediately clear if the cops were using lights or sirens.

By the time the officers caught up with the truck, the driver, a 34-year-old man, had careened through the crowded crosswalk, smashing into the pedestrians, the source said.

The truck stopped only when it smacked into at least five cars and a traffic-light post.

Police said the driver, who was not immediately identified, would be tested to see if he had been drinking.

“The only thing I remember seeing was this white truck barreling down, no brakes or anything,” said passerby Debbie Stephens.

“The weirdest part about it was there was no screeching,” she said. “Just Boom! Boom! Boom! and then silence and then cops everywhere.”

“It was flying like an airplane, ” said another witness, Artura Samba.

The pickup had been heading east on 72nd Street and ran the light at Broadway, police said.

“I saw the white pickup roll onto the top of a beige pickup and go airborne,” said John Marshall, who was driving by with his family.

Alex Glen, who had crossed the intersection moments before the bloody crash, said: “I could hear the tires screeching all the way down the road – he must have been going 80 mph on the wrong side of the road.”

Joe Rand, who also had just crossed the street, said the driver “hit the pedestrian head-on.

“The pedestrian was thrown around the back of the truck. Instead of flying up, he flew around it.”

Three people were taken to Bellevue Hospital for multiple injuries, and six others were taken to St. Luke’s and Roosevelt hospitals.

All were in stable condition.

Police withheld the names of the victims pending notification of their families.